This invention relates to polyurethane compositions and their method of preparation. More particularly, this invention relates to coldseal, aqueous-based polyurethane adhesive compositions prepared by controlled chain extension of urethane prepolymers with an excess of water.
The reaction of water with urethane prepolymers to prepare polyurethane foams is well known. The water reacts with isocyanate groups in the prepolymer to provide an unstable carbamic acid which decomposes to an amine with elimination of carbon dioxide. The amine, in turn, reacts with other isocyanate groups in the prepolymer to form urea linkages and effect cross-linking or chain extension of the prepolymer. The resultant viscosity increase results in entrapment of the liberated carbon dioxide with eventual formation of the desired foam.
Certain reactions of water with urethane prepolymers to provide adhesive compositions containing polyurethane elastomers are also known. In the preparation of such compositions, foaming due to the combination of carbon dioxide evolution and viscosity increase is highly undesirable. In order to minimize or avoid foaming, the water has been used in controlled small amounts, approximately equal to or less than an equivalent amount based on the isocyanate content of the prepolymer, and the reaction has been conducted in the presence of a relatively large quantity of an inert organic solvent. The reaction tends to be lengthy, due to the relatively small amount and low concentration of water and the adhesive composition is produced as an organic solvent solution.
From a process standpoint, the above method is unsatisfactory, not only with respect to reaction time but also with respect to the cost and handling of large volumes of organic solvents. From a product standpoint, the organic solvent base of the composition can be undesirable with respect to both cost and environmental and safety factors.